Coffee is the world's most widely traded tropical agricultural commodity, with about $30 billion dollars spent worldwide on the product and about one billion dollars spent annually on the energy required to roast the beans. Green coffee beans must be roasted before they are used in all forms of the coffee beverage. Roasting the beans is accomplished using a variety of heating methods and at a variety of scales, ranging from mass-market industrial roasters running continuously to the single batch home roaster processing a couple of batches a week. To place the economic impact of global coffee roasting in perspective, the cost of energy required to roast the world's yearly supply is about $1×109 (calculated using the average 2011 consumer cost of electricity in the US, $0.1/kW-h), hence both economic and quality optimization is of interest.
Controlling the roast time and temperature profile results in a range of roast levels from light to dark, greatly affecting the style, flavor, and aroma of the resulting coffee beverage. Terminating the roasting process at just the right time allows the roaster (most often, a human operator) to achieve the desired darkness of the roast and its accompanying flavor profile, and hence is one of the key roast parameters. Several metrics can be monitored (time, color, aroma, bean volume, bean temperature), using process measurement instrumentation or by the person conducting the roast, to indicate the degree of roasting and ultimately to determine when to terminate the roast.
The roasting process can also be monitored by ear, by listening for events known collectively as the “first crack” and “second crack,” which also signify the progression of the roast. During the roasting process, the human operator may listen for two temperature thresholds called “cracks.” At about 200-220° C. (392-428° F.), beans will emit a cracking sound much like popcorn does when it pops, only much quieter. This point is called the “first crack,” marking the beginning of light roasts. When the beans are at about 224-245° C. (435-473° F.), they emit a “second crack.” During the first and second “cracks,” pressure inside the bean has increased to the point where the structure of the bean fractures, rapidly releasing gases, thus an audible sound is emitted. By listening for such cracks, the operator may terminate the roasting process to achieve the desired darkness of the roast and its accompanying flavor profile.
Typically though, roasters, especially large commercial roasters, will often generate enough noise internally that the operator can no longer hear the first or second cracks, which are strong indicators of the roasting process. Hence, the operator may under or over roast the beans due to the inability of hearing these cracks thereby leading to an inconsistent product. Furthermore, over roasting the beans results in wasted energy.
Hence, the current roasting process of coffee beans involves an inconsistent roasting of the beans as well as wasted energy when the beans are over roasted.